1. Field of the Invention
The present invention relates to a method for controlling a rotating storage unit such as a magnetic disk storage unit, and more particularly to a control method suitable for parallel-transferring data on a plurality of tracks.
2. Description of the Prior Art
In a conventional magnetic disk storage unit, one magnetic disk is selected so that data on one track at a time is transferred. A data transfer rate between the magnetic disk storage unit and a host system is limited by a linear recording density and a rotation speed. On the other hand, performance of CPU's have been rapidly improved. Transfer capabilities of a channel have also been improved. As a result, a transfer capability of the magnetic disk storage unit is considerably lower than that of the channel. In order to maximize the transfer capability of the channel, it is necessary to substantially improve the transfer rate of the magnetic disk storage unit. In Japanese Patent Unexamined Publication No. 55-108915, a plurality of magnetic heads coupled to one accessing mechanism is simultaneously selected so that data is parallelly transferred from or to a plurality of tracks. When this method is applied to a known fixed record length type magnetic disk unit, the magnetic disk is sectioned so that each of the tracks has a record of data, and when data is to be read, the data is parallelly read from the sectors of the tracks and stored into a buffer memory and the stored data is transferred to the channel, and when data is to be written, the data to be written into the sectors of the tracks is transferred from the channel to the buffer memory and stored therein. The stored data is parallelly written onto the tracks.
There may exist a defect area on the medium at which information cannot be recorded or reproduced. In the fixed record length type magnetic disk storage unit, an alternative sector area of several record lengths is usually provided at an end part of the track, and if there is a defective area in a record, an alternative sector is assigned as the alternative sector area in the same track.
When the alternative area is assigned in this manner, the following problem will arise. If a defect in a sector of a track is detected during the readout of data by defect identification information recorded in that sector, data is read from the alternative sector area of that track, and then data of the tracks to be parallelly read is read from the sector next to the defective sector. Accordingly, if the defect is detected in one sector, the data of the next sector is read after the magnetic disk has been rotated one revolution. When the defect of a sector is detected while data is written, the data is written into the alternative sector area and then the data is written into the next sector. Thus, once the defective sector is detected, the data is written into the next sector after the magnetic disk has been rotated one revolution. In this manner, when the defective sector is detected, a waiting time is required for accessing the alternative sector and the input/output time significantly increases.